This invention relates to light emitting devices including a light emitting diode in combination with a phosphor material. Light emitting diodes (LEDs) are well-known solid-state devices that can generate light having a peak wavelength in a specific region of the visible spectrum. Early LEDs emitted light having a peak wavelength in the red region of the light spectrum, and were often based on aluminum, indium, gallium and phosphorus semiconducting materials. More recently, LEDs based on Group III-nitride where the Group III element can be any combination of Ga, In, Al, B, and Tl have been developed that can emit light having a peak wavelength in the green, blue and ultraviolet regions of the spectrum.
Since these GaN based devices have a relatively short peak wavelength, the blue, green or UV light (the “primary light”) generated by the GaN-based LEDs is often converted to produce light having a longer peak wavelength (the “secondary light”) using a process known as photoluminescence emission. The photoluminescence emission process involves the absorbance of the primary light by a photoluminescent phosphor. The absorbed light excites the luminescent centers of the phosphor material, which then emit the secondary light upon relaxation. The peak wavelength of the secondary light will depend on the phosphor material. The type of phosphor material can be chosen to yield secondary light having a selected wavelength. Accordingly, the phosphor material may emit light at the wavelength of the overall device light color or the device light color may be achieved via a combination of emitted primary LED light and converted secondary phosphor light. As used in this description, the term phosphor material includes a luminescent material in loose or packed powder form and in solid crystalline body form).
With reference to FIG. 1. a prior art light emitting device 10 is shown. The light emitting diode 10 includes an LED 12 that generates blue primary light when activated. The LED 12 is positioned on a reflector cup lead frame 14 and is electrically coupled to leads 16 and 18. The leads 16 and 18 provide electrical power to the LED 12. The LED 12 is covered by a region 20 of phosphor material. The LED 12 and the phosphor region 20 are encapsulated by a lens 22. The lens 22 is typically made of an optically transparent epoxy, or silicone.
In operation, electrical power is supplied to the LED 12 to activate the semi-conducting layers, e.g. Group III-nitride. When activated, the LED 12 emits the primary light, e.g., blue light, away from the top surface of the LED 12. The emitted primary light is absorbed by the phosphor region 20. The phosphor region 20 then emits secondary light, i.e., the converted light having a longer peak wavelength, in response to absorption of the primary light. The secondary light is emitted randomly, or isotropically in various directions by the phosphor region 20. Some of the secondary light is emitted away from the GaN die 12, propagating through the lens 22 and exiting the light-emitting device 10 as output light. The lens 22 directs the output light in the general direction indicated by arrow 24.
According to the prior art design of FIG. 2, a light emitting device structure is shown in which a light emission chip 100 is placed in a V-shaped slot (generally called a cup) of a first electrode supporting frame 102. After connecting the conductors (not shown), a first resin 103 is coated on the V-shaped slot with its height slightly larger than the thickness of the light emission chip 100, a second resin 104 is coated on the first resin 103 after it has cured. A phosphor material capable of changing the length of light waves is mixed with the second resin 104. After the second resin 104 has dried, a third resin 105 is applied as a sealing enclosure for the entire structure to form a finished LED product.
Alternatively, light emitting devices have been provided wherein phosphor and LED are mounted to a top surface of a printed circuit board. According to the prior art design shown in FIG. 3, it is taught that the deposition of a transparent spacer 200 over and around the LED 202, separating a substantially uniform thickness layer of phosphor material 204 from the LED 202 will eliminate annular rings. It is also stated that the transparent spacer 200 can be exactly level with the top of the LED 202, such that the layer of phosphor material 204 is of a uniform thickness above the LED. The LED 202 is mounted on a printed circuit board 206. This too suffers from disadvantages including the failure to position phosphor advantageously close to the light emitting region of the LED.
In summary, the prior art has primarily combined phosphor material with an LED by dispersing the phosphor material in a matrix surrounding the LED (“conformal coating”) or in a layer disposed above the LED (“remote coating”).
Importantly, LEDs, particularly GaN type, can be divided into two sub-categories. Moreover, most GaN LEDs available today are provided either on a sapphire substrate layer or a silicon carbide substrate layer. For reasons known to the artisan skilled in the manufacture of GaN LEDs in these categories, silicon carbide substrated GaN LEDs typically have a thickness of nearly 2.5 times that of sapphire substrated LEDs. As utilized herein, the terms “thickness” and “height” reflect the dimension of the LED in the direction of arrow 301 in FIG. 4.
The present invention helps to address problems in conformal coating of a relatively thick substrate (e.g. silicon carbide) LED.